This invention relates to integrated circuit devices and, more particularly, to microprocessors.
The desires to save as much circuit board space as possible and to integrate as much of a circuit function on a single chip as possible have resulted in placing some microprocessor-associated functions on the integrated circuit with the microprocessor. One of these MPU-associated functions is the oscillator for generating the clock of the microprocessor. Two types of MPU clock oscillators are in common use for differing applications, the RC oscillator which is relatively less expensive to implement and the crystal oscillator which is inherently more stable. Consequently, it is desirable to provide the option for choosing either type of oscillator on the integrated circuit.
Typically, the frequency of a crystal oscillator is chosen by the selection of the crystal whereas the frequency of an RC oscillator is chosen by the selection of the resistor of the RC pair. If both options are to be provided on an MPU chip, this implies that two external pins must be provided for the RC oscillator and two external pins must be provided for the crystal oscillator. Whereas it is desirable to offer both oscillator options on the chip, it is necessary to only provide one for a given customer's requirement. Consequently, if both options are provided, the customer is provided with two pins that are unnecessary for his application. If only the two external pins that the customer requires are provided, an advantage in providing a smaller package results. When the integrated circuit has an on-board read only memory control store (ROM) which will be programmed for a particular customer for a given application, it is known in advance which oscillator circuit is desired. This fact affords the opportunity to alter the integrated circuit during the manufacture thereof so that only the two external pins required for the oscillator of the type required by the customer are provided to that customer. Although providing both oscillator options facilitates the manufacturing advantage of only making one type of integrated circuit, it has the disadvantage of requiring more chip space than would be required if only one oscillator were available. Accordingly, it is an object of the instant invention to provide an integrated circuit with plural oscillator types thereon, selectable at the time of manufacture, utilizing a minimum of chip area, and providing a minimum of external pins.